Ethylene oxide is commercially produced by the epoxidation of ethylene over silver-containing catalyst at elevated temperature. Considerable research efforts have been devoted to providing catalysts that increase the efficiency, or selectivity, of the process to ethylene oxide.
The manufacture of ethylene oxide by the reaction of oxygen or oxygen-containing gases with ethylene in the presence of a silver catalyst is an old and developed art. For example, U.S. Pat. No. 2,040,782, patented May 12, 1936, describes the manufacture of ethylene oxide by the reaction of oxygen with ethylene in the presence of silver catalysts which contain a class of metal promoters. In U.S. Pat. No. Re. 20,370, dated May 18, 1937, Leforte discloses that the formation of olefin oxides may be effected by causing olefins to combine directly with molecular oxygen in the presence of a silver catalyst. From that point on, the prior art has focused its efforts on improving the catatyst's efficiency in producing ethylene oxide.
In characterizing this invention, the terms "conversion", "selectivity", and "yield" are employed as defined in U.S. Pat. No. 3,420,784, patented Jan. 7, 1969, at column 3, lines 24-35 inclusive. This definition of "selectivity" is consistent with that disclosed in U.S. Pat. No. 2,766,261 at column 6, lines 5-22, and U.S. Pat. No. 3,144,916, lines 58-61. The definitions of "yield" and "conversion" have more varied meaning in the art and are not to be employed as defined, for example, in the aforementioned U.S. Pat. No. 2,766,261. The terms "efficiency" and "selectivity", as used throughout the specification and claims are intended to be synonymous.
Silver catalysts employed in the manufacture of ethylene oxide have undergone significant changes since their initial period of development. As reported by the art, silver particles were first deposited upon support materials with little attention being paid to support properties, such as surface area, pore volume and chemical inertness. As the art evolved, there developed special technologies related to carriers or supports containing silver that were more effective for the reaction of ethylene with oxygen to produce ethylene oxide. Today, most supports for the silver catalyst.sub.s are shaped particulate materials which can be loaded in the interior of a reactor wherein the reacting gases and the gaseous products of the reaction are capable of flowing in and about these particulate materials to pass through the reactor and be recovered. The size and shape of the support are variable factors and the particular size and shape selected are peculiar to the reactor employed, the gas flow required, and the pressure drop across the reactor, with other factors also being considered.
The carriers that have been employed are typically made of inorganic materials, generally of a mineral nature. In most cases, the preferred carrier is made of alpha-alumina, such as has been described in the patent literature: see for example, U.S. Pat. Nos. 2,294,383; 3,172,893; 3,332,887; 3,423,328; and 3,563,914.
The carriers which are employed for the manufacture of most, if not all, commercially employed ethylene oxide catalysts are produced by companies who do not produce such catalysts. As a rule, the methods of making such carriers are trade secrets of significant value to the carrier manufacturers. Consequently, the catalyst manufacturer cannot know how the carrier is made. Critical to making a carrier which proves uniquely desirable for the manufacture of a successful catalyst can be a number of factors, such as the purity and other physical/chemical properties of raw materials used to make the carrier and the method by which the carrier is made.
The silver that is deposited on these carriers is thought to be in the form of small particles because that is all that can be seen by current microscopic techniques. The patent literature indicates that the size of the silver is a factor in the effectiveness of the catalyst and in most cases fine particle silver is obtained utilizing the standard processes in the art; see, for example, U.S. Pat. Nos. 2,554,459; 2,831,870; 3,423,328 (specifies that silver particles of 150-400 Angstroms are employed); 3,702,259 (disclosed a preparation procedure for forming silver particles less than 1 micron in diameter) and 3,758,418 (discloses silver particles having a diameter less than 1000 Angstroms). Improvements in microscopic examinations of silver catalysts enable the observation that the particle size ranges to even smaller values.
The deposition of silver onto the carrier can be achieved by a number of techniques but the two techniques which are most frequently employed involve, in one case, the impregnation of the support with a silver solution followed by heat treatment of the impregnated support to effect deposition of the silver on the support and, in the other case, the coating of the silver on the support by the precipitation of silver or the preformation of silver into a slurry such that the silver particles are deposited on the support and adhere to the support surface when the carrier or support is heated to remove the liquids present. These various procedures are exemplified in various U.S. Pat. Nos. such as 2,773,844; 3,207,700; 3,501,407; 3,664,970 (see British Patent 754,593) and 3,172,893.
The surface area provided by the support has been the subject of considerable interest in the development of silver catalysts. Disclosures concerning the surface area of the catalyst carrier can be found in U.S. Pat. No. 2,766,261 (which discloses that a surface area of 0.002-10 m.sup.2 /gm is suitable); U.S. Pat. No. 3,172,893 which depicts a porosity of 35-65% and a pore diameter of 80-200 microns); U.S. Pat. No. 3,725,307 which depicts a surface area of less than 1 sq.m/gm and an average pore diameter of 10-15 microns): U.S. Pat. No. 3,664,970 (which utilizes a support having a minimum porosity of about 30%, at least 90% of the pores having diameters in the range of 1-30 microns, and the average of such diameters being in the range of 4-10 microns); and U.S. Pat. No. 3,563,914 which utilizes a catalyst support having a surface area of less than 1 sq.m/gm, a volume of 0.23 ml/gm and a particle size between 0.074 and 0.30 mm). Low surface area, inert alpha-alumina is favored by the prior art.
It has been known for a long time that impurities present in the catalyst and/or the gas phase can materially impact upon the reaction. In the early development of the art, there were no techniques available for identifying or measuring such impurities. Consequently, one could not isolate the role that such impurities played. However, even in the earliest periods of the development of the art, the use of alkali metals as promoters for the silver catalyzed production of ethylene oxide was extremely well known in the art. U.S. Pat. No. 2,177,361, issued October 1939, has a teaching of the use of alkali metals in silver catalysts. U.S. Pat. No. 2,238,471 discloses that lithium is very desirable as a promoter but that potassium and cesium are detrimental when used in amounts of essentially 10% by weight of potassium hydroxide or cesium hydroxide to the silver oxide employed in making the catalyst. Later, U.S. Pat. No. 2,404,438 states that sodium and lithium are effective promoters for this reaction. Essentially the same teaching can be found in U.S. Pat. No. 2,424,084. U.S. Pat. No. 2,424,086 generalizes about alkali metals as promoters and specifies sodium in particular. In U.S. Pat. No. 2,671,764 (the Sacken sulfate patent), the patentees believe that alkali metals in the form of their sulfates are effective as promoters for such silver catalysts. In particular, the patentees state that sodium, potassium, lithium, rubidium or cesium sulfates may be used as promoters.
U.S. Pat. No. 2,765,283 describes the pretreatment of a support with a dilute solution of a chlorine-containing compound and indicates that such chlorine compounds should be inorganic. Particular illustrations cited of suitable inorganic chlorine compounds included sodium chloride, lithium chloride and potassium chlorate. This patent specifies that the amount of the inorganic chlorine-containing compound which is deposited on the catalyst support is from 0.0001% to 0.2% by weight based on the weight of the support, U.S. Pat. No. 2,615,900 to Sears describes the use of metal halide in the treatment of the supported catalyst and specifies that such halides can be of alkali metals such as lithium, sodium, potassium and cesium. The metal halide is present in the range of 0.01% to 50% based upon the weight of metallic silver. The patent also specifies that mixtures of the individual metal halides generally classified in the patent may be used to advantage to enhance the break-in period of a new catalyst composition while at the same time maintaining a moderate but steady activity of the catalyst over an extended period of time during normal operation. Thus, one particular metal halide treated catalyst would provide a short-term high initial activity whereas another of the metal halides would provide a longer term moderate activity for the catalyst. This patent takes the position that the metal halides which are provided in the catalyst serve to inhibit the combustion of ethylene to carbon dioxide and thus classifies these materials as catalyst depressants or anticatalytic materials.
U.S. Pat. No. 2,709,173 describes the use of a silver catalyst for making ethylene oxide in which there are provided simultaneously with the introduction of silver to the solid support, any of the alkali metal halides such as lithium, sodium, potassium, and rubidium compounds of chlorine, bromine and iodine, to enhance the overall production of ethylene oxide. The patent specifies small amounts "of less than about 0.5% are desirable." In particular, the patent emphasizes "proportions of alkali metal halide within the range of about 0.0001 to about 0.1" are most preferred. The patent states that "although the preferred catalyst composition contains a separate promoter it is not always necessary since during preparation of the catalyst the alkali metal halide may be converted to some extent to the corresponding alkali metal oxide which acts as a promoter." U.S. Pat. No. 2,766,261 appears to draw from the teachings of U.S. Pat. No. 2,238,474 in that cesium and potassium are said to be detrimental in silver catalysts; sodium and lithium are suggested as useful promoters. However, U.S. Pat. No. 2,769,016 finds that sodium, potassium and lithium are promoters when used in the silver catalysts. This latter patent also recommends the pretreatment of the support with dilute solutions of sodium chloride, lithium chloride or potassium chlorate.
U.S. Pat. No. 2,799,687 to Gould, et al., states that the addition of metal halides within the range described by Sears in U.S. Pat. No. 2,615,900 is not productive of optimum results. This is said to be especially true in the case of alkali metal halides, particularly the chloride and fluoride of sodium and potassium. The patentees recommend that the inorganic halide component of the catalyst be maintained within the range of 0.01-5 weight percent, preferably 0.01 to 0.1 weight percent, based on the weight of the "silver oxidative catalytic component," i.e., the silver salt transformed into elemental silver. U.S. Pat. No. 3,144,416 mentions a variety of metals as promoters and one of them is cesium. U.S. Pat. No. 3,258,433 indicates that sodium is an effective promoter. U.S. Pat. No. 3,563,913 recommends the use of alkali metals such as lithium compounds as promoters. The preferred amount of promoting material is said to be about 0.03 to 0.5%, by weight of metal oxide based on the weight of the support. U.S. Pat. No. 3,585,217 states that alkali metal chlorides "are known to counteract the formation of carbon dioxide" and "may be incorporated into the catalyst." U.S. Pat. No. 3,125,538 discloses a supported silver catalyst containing a coincidentally-deposited alkali metal selected from among potassium, rubidium and cesium in a specified gram atom ratio relative to silver. The weight of silver is preferably 2-5% by weight of the catalyst. The patentees characterize this catalyst as being especially suitable for the reaction of nitric oxide with propylene. This same catalyst is produced inherently by the processes of the examples of U.S. Pat. No. 3,702,259, as discussed previously, which patent promotes their use or making ethylene oxide. U.S. Pat. Nos. 3,962,136 and 4,012,425 also disclose that same catalyst as being useful for ethylene oxide production. U.S. Pat. No. 3,962,136 describes the coincidental deposition of alkali metal with the silver on the support, the alkali metals being present in their final form on the support in the form of an oxide in which the oxide consists of cesium, rubidium or mixtures of both, optionally combined with a minor amount of an oxide of potassium. The amount of such oxide is from about 4.0.times.10.sup.-5 gew/kg to about 8.0.times.10.sup.-3 gew/kg of total catalyst. U.S. Pat. No. 4,356,312 describes the use of the same catalyst. U.S. patent application Ser. No. 317,349, filed Dec. 21, 1972, which is a parent to U.S. Pat. Nos. 3,962,136 and 4,010,115 and others, contains some interesting data deserving of comment. According to example 2 which contains some comparative experiments, there is described the manufacture of a catalyst which contains 310 parts per million by weight of coincidentally-added potassium and that catalyst when employed as an ethylene oxidation catalyst was found to be inactive for the production of ethylene oxide.
U.S. Pat. No. 4,207,210 (cortes. Belgium Patent 821,439, based upon British Patent Specification 1,489,335) discloses that a catalyst can be made that is equivalent to that produced in the so-called parent applications cited in U.S. Pat. Nos. 3,962,136, 4,012,425, and 4,010,115 by using a sequential procedure by which the alkali metal is supplied to the support. Thus, the criticality in the method of deposition of alkali metal in the catalyst appears doubtful in the face of that type of disclosure and the disclosure of U.S. Pat. Nos. 4,033,903 and 4,125,480 which describe subjecting used silver-containing catalysts to a post-addition of one or more of potassium, rubidium or cesium. Apparently, such treatment regenerates the catalyst's ability to enhance selectivity to ethylene oxide. Another patent which tends to indicate that a post-addition of alkali metal such as cesium gives results equivalent to either pre-addition or simultaneous addition is U.S. Pat. No. 4,066,575.
German Offenlegungsschrift 2,640,540 discloses in its examples a silver catalyst for ethylene oxide production containing sodium and either potassium, rubidium or cesium.
Japanese Application Publication Disclosure No. 95213/75 is directed to a process for producing ethylene oxide using a catalyst composition comprising silver, barium, potassium and cesium in specified atomic ratios. Table I of this disclosure summarizes the efficiencies achieved with the various catalyst compositions of the examples.
U.S. Pat. No. 4,039,561 discloses a catalyst for preparing ethylene oxide containing silver, tin, antimony, thallium, potassium, cesium and oxygen in specified atomic ratios.
Belgium Patent 854,904 discloses silver catalysts containing various mixtures of sodium and cesium. U.K. Patent Application 2,002,252 discloses, in Table 2, supported silver catalysts containing various mixtures of cesium and thallium, some of which additionally contain potassium or antimony. U.S. Pat. No. 4,007,135 broadly discloses (in column 2, lines 25-30) silver catalysts for alkylene oxide production containing silver "together with a promoting amount of at least one promoter selected from lithium, potassium, sodium, rubidium, cesium, copper, gold, magnesium, zinc cadmium, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium and barium . . . ". U.S. Pat. Nos. 3,844,981 and 3,962,285 disclose catalysts and processes for epoxidizing olefins in the presence of a multimetallic component. The catalyst in the U.S. Pat. No. 3,962,285 is said to comprise a minor amount of one or more of palladium, ruthenium, rhenium, iron and platinum with a major amount of silver. The U.S. Pat. No. 3,844,981 discloses the preparation of the catalyst from a decomposible salt of group 7b, 1b or the iron group of group 8 of the Periodic Table of the Elements. Preferably, the salt is selected from the group of gold, copper, rhenium, manganese and iron salts. While the patentee contemplates that these metals are in the metallic state, oxidation during epoxidation conditions may occur with one or more of these metals, e.g., rhenium, to form oxyanions containing the metal.
U.S. Pat. No. 2,605,239 discloses the use of beryllium oxide as a promoter. Other promoter metals such as copper, aluminum, manganese, cobalt, iron, magnesium, gold, thorium, nickel, cesium and zinc are suggested. These promoter metals are to be incorporated into the catalyst by mechanical mixture or coprecipitation.
European Patent Publication No. 0003642 discloses, in Table 3, silver-containing catalysts which include mixtures of potassium and cesium, and a catalyst containing sodium and cesium.
Belgium Patent 867,045 discloses supported silver catalysts containing what is referred to as an effective proportion of lithium and a substantially lesser amount of alkali metal selected from among cesium, rubidium and/or potassium.
Belgium Patent 867,185 discloses supported silver catalysts for ethylene oxide production containing a specified amount of potassium and at least one other alkali metal selected from rubidium and cesium.
United Kingdom Patent No. 2,043,481, commonly assigned, describes the use of a synergistic combination of cesium and at least one other alkali metal in combination with silver on an inert support to provide catalysts which were superior to those known to the art at that time. Such catalysts have been widely employed commercially. The alkali metal components are provided to the support by a variety of ways. The alkali metal can be supplied to the support as a salt and many salts of the alkali metals are described. Specific illustration is made of the use of alkali metal sulfates as one of many usable alkali metal compounds.
European Patent Application 85,237 describes an ethylene oxide catalyst wherein the applicants believe they "chemically absorbed" by alcohol wash, cesium and/or rubidium onto the catalyst support.
Japanese patent application Kokai 56/105,750 discloses, among other things, ethylene oxide catalysts containing cesium molybdate or cesium tungstate or cesium borate. The catalyst is stated to have an alumina carrier having a sodium content of less than 0.07 weight % and mainly consisting of alpha-alumina having a specific surface area of 1 to 5 sq. m./gm. The carrier is impregnated with decomposible silver salt solution containing alkali metal boron complex, alkali metal molybdenum complex and/or alkali metal tungsten complex. No examples of mixtures of anions are disclosed. Japanese patent application Kokai 57/21937 discloses thallium-containing catalysts in which the thallium may be borate or titanate salt.
European patent application 247,414, published Dec. 12, 1987, discloses catalysts containing alkali metal and/or barium which may be provided as salts. The salts include nitrates, sulfates, and halides. U.S. Pat. Nos. 4,761,394 and 4,766,105 disclose catalysts containing a rhenium component, e.g., rhenium oxide, rhenium cation or rhenate or perrhenate anion. An example of a catalyst made from silver oxalate with cesium hydroxide, ammonium perrhenate, and ammonium sulfate is disclosed in the '394 patent. Numerous examples of silver catalysts containing cesium, rhenate and co-promoter salts are presented in the '105 patent. Experiments 7-1, 7-2, 7-3, 7-4, 7-12 and 7-27 as reported in the '105 patent are summarized below.
______________________________________ Other Experi- Ag, Re, Compo- Initial Initial ment % Cs, ppm ppm nent, ppm S.sub.40, % T.sub.40, .degree.C. ______________________________________ 7-1 14.3 236 0 None 80.0 242 7-2 13.9 360 186 None 80.6 241 7-3 14.2 438 372 None 81.9 248 7-4 13.3 405 186 (NH.sub.4).sub.2 SO.sub.4, 83.1 259 32(S) 7-12 13.5 328 186 KMnO.sub.4, 80.8 242 55(Mn) 7-27 14.3 293 + 7Li 186 (NH.sub.4).sub.2 SO.sub.4, 82.4 245 32(S) ______________________________________
S.sub.40 and T.sub.40 are defined in the patent and are the efficiency and temperature at 40 percent oxygen conversion as determined at about 16.+-.4 hours.
Several phenomena appear to be discernible from these data. Rhenate appears to enhance efficiency, especially in the presence of certain "co-promoters" such as sulfate anion. Furthermore, when the amount of rhenate is increased or a copromoter is used which increases efficiency, the temperature required for 40 percent oxygen conversion ("T.sub.40, .degree.C.") also appears to increase in most instances. The presence of 55 ppm Mn as KMnO.sub.4 in Experiment 7-12 appears to have little, if any, effect on S.sub.40 (selectivity at 40 mol % oxygen conversion) or T.sub.40.
While improved efficiencies of conversion to ethylene oxide are desirable, the concommitant increase in temperature (i.e., loss of activity) can be troublesome for a commercially-viable catalyst. Commercial ethylene oxide plants are typically operated to provide a desired balance of productivity and efficiency. Less active catalysts are thus operated at higher temperatures to achieve desired productivity. However, the upper temperature range of the catalyst is limited. Consequently, catalysts that have high initial temperatures for a given conversion rate may have shorter useful lives. Not only is catalyst a major expense to the ethylene oxide plant owner, but also, the plant must be shut down for substantial periods of time to discharge the old catalyst and charge new catalyst to the typical tubular, fixed bed ethylene oxide reactors. Hence, without a useful lifetime, e.g., two years or more, the benefit of any enhanced efficiency is quickly lost in catalyst replacement costs and plant shut-down time.
U.S. patent applications Ser. Nos. 18,808, filed Feb. 20, 1987, now abandoned; 640,269, filed Aug. 13, 1984, now abandoned; and 251,573 and 251,814, both filed Oct. 3, 1988, M. M. Bhasin, disclose silver-containing, supported catalysts for ethylene oxide production containing cesium salts of oxyanions having an atomic number of at least 15 to 83 and being from groups 3b through 7b and/or groups 3a through 7a of the Periodic Table of the Elements (as published by The Chemical Rubber Company, Cleveland, Ohio, in CRC Handbook of Chemistry and Physics, 46th Edition, inside back cover). The oxyanions include, by way of illustration, sulfate, phosphates, manganates, titanates, tantalates, molybdates, vanadates, chromates, zirconares, polyphosphates, tungstates, cerates, and the like. The following table summarizes several examples contained in the '573 patent application
______________________________________ Other Example Cs Salt, Salt, Efficiency, Temper- No. Ag, % Amount Amount % ature, .degree.C. ______________________________________ 15 13.09 0 0 73.5 238 23 13.55 CsMnO.sub.4, 0 79.7 240 0.0101% 29 13.25 CsMnO.sub.4, KMnO.sub.4, 79.6 249 92 ppm 27 ppm Cs K 30 13.25 CsMnO.sub.4, KMnO.sub.4, 76.0 254 95 ppm 28 ppm Cs K H.sub.2 SO.sub.4, 48 ppm 34 14.1 CsMnO.sub.4, KMnO.sub.4, 59.6 281 51 ppm 152 ppm Cs K ______________________________________
Examples 15, 23, 29, and 30 were conducted under oxygen process conditions and Example 34 was conducted under air process conditions. Oxygen and air process conditions are generally described herein.
Manganese has been proposed for use in catalysts for other applications. For instance, United Kingdom patent application 2,095,242A, published Sep. 29, 1982, discloses the oxychlorination of alkanes in the presence of a solid particulate catalyst composition comprising (1) metallic silver and/or a compound thereof and (2) one or more compounds of manganese, cobalt or nickel. Japanese patent application Kokai 57/136941, published Aug. 24, 1982, discloses catalysts for the decomposition of ozone. The catalyst appears to be made by adding 0.1 to 20 weight percent of silver and 1 to 20 weight percent of cobalt oxide (calculated as atomic percent of cobalt) to manganese dioxide. Imamura, et al., in "Oxidation of Carbon Monoxide Catalyzed by Manganese-Silver Composite Oxides", J. of Catalysis, vol. 109, pp 198-205 (1988) and "Effect of Samarium on the Thermal Stability and Activity of the Mn/Ag Catalyst in the Oxidation of CO", J. of Catalysis, vol. 115, pp 258-264 (1989) disclose manganese-silver catalysts for the catalytic oxidation of carbon monoxide. U.S. Pat. No. 4,800,070 is directed to the catalysis of a nitrate-nitrite system for the separation of oxygen from oxygen-containing gases such as air. The catalyst comprises transition metal oxide selected from the group consisting of oxides of manganese, ruthenium, rhenium, osmium, rhodium, iridium and mixtures thereof.
Methods are sought to enhance the activity and/or stability of silver-containing, supported ethylene oxide catalysts which have been promoted to enhance efficiency, which while providing desirable efficiencies, are typically less active and must be operated at higher temperatures to be useful in commercial production facilities. These high temperatures can unduly-shorten the catalyst life such that the catalysts are unattractive for commercial facilities.